In a recirculating memory, data is injected into a closed loop at a high rate, and recirculates around the loop in a continuous unidirectional motion. The entire group of data stored in the loop may be repeatedly read from the loop at a period equal to the time taken for the data to complete one cycle around the loop. In this way, a group of data may be stored in the recirculating memory, and the data may be read from the memory at a slower rate since the data will continue to circulate in the loop for some extended period of time, thus allowing different portions of the data to be sampled after each circulation.
Such systems are quite useful in applications where data is generated at a rate faster than it can be accepted by a data procesor. By installing a recirculating memory between the data source and the data processor, the data can be supplied to the data processor at the rate necessary for the data processor to analyze the data without loosing any of it. Another use of recirculating memory devices is for the temporary storage and retrieval of broadband microwave signals in electronic counter-measures (ECM) used to jam radar signals or to project false radar images.
Many of these systems use coaxial cables or acoustic wave guides rather than using a loop for the storage of signals modulated onto the microwave carriers. Such devices are basically delay lines in which a time delay is produced because of the time a signal takes to travel through the delay lines from the input end to the output end. In addition to the direct output of a given input signal, a portion of that signal will be reflected and will propagate from the output end back to the input end, where it is reflected to the output end again, resulting in a second output signal identical to the first output signal, although smaller in amplitude. The data pulse will continue to be reflected and outputted from the delay lines for some period of time, resulting in a number of identical pulses, equidistantly spaced, with decreasing amplitude.
The coaxial cable delay line is the most common type of delay line, and microwave signals may be stored in coaxial cables for some period of time. Coaxial cable may be used with directional couplers, which couple a secondary system to a wave traveling in a particular direction in the primary transmission system. When used as a recirculating memory, however, coaxial delay lines have several disadvantages. The first disadvantage is the limited bandwidth of coaxial cable, making coaxial cables useless at high frequencies and with short pulses.
At frequencies above 100 MHZ, coaxial cable is subject to severe losses, and high frequencies will thus not be transmitted accurately. In addition, if the pulse being transmitted is of extremely short duration, e.g., one nanosecond, it will be degraded and spread out rather than remaining sharp. This limits the number of pulses which can be transmitted close together, and, consequently, the information-carrying capability of the coaxial cable.
A second disadvantage of coaxial cable is that it is susceptible to electromagnetic interference, particularly when the frequencies being transmitted are relatively high. Finally, in order to have a coaxial cable delay line with a sufficiently long delay time, a considerable length of coaxial cable is necessary. Such delay lines are quite bulky, and also fairly expensive.
A second technique utilized to create delay lines and recirculating memories is through the use of acoustic delay lines. There are two types of acoustic delay lines: bulk-wave devices, and surface-wave devices. Bulk-wave devices use the principle of compression and rarification of the bulk material, and have input and output transducers at the ends of the bulk material. Bulk-wave devices unfortunately require large bias voltage and thus present a heat dissipation problem, so that only pulsed operation of bulk-wave devices is feasible.
Surface-wave devices operate with acoustic surface waves, and utilize charge carriers in a thin film of silicon placed adjacent to an insulating piezoelectric crystal. Surface acoustic wave memories operating at UHF frequencies have been developed. The main disadvantage of such acoustic wave memories is that their upper operational frequency limit is approximately 1 GHZ, while it is desirable to have a recirculating memory operable at higher frequencies.
Attempts to develop a fiber optic recirculating memory have been unsuccessful, in part because of the lack of an optical directional coupler. One attempt to create such a system is disclosed in U.S. Pat. No. 4,136,929, to Suzaki, entitled "Apparatus for Generating Light Pulse Train." The object of this invention was to produce a pulse train with identical pulses as an output, with a single pulse as the input to the system. The most interesting embodiment of this invention is shown in FIG. 1A, which has a fiber running through a coupling device with the ends of this fiber being the input and output, and a loop fiber also running through the coupler device and being optically coupled with the input-output fiber.
This concept is rendered impractical by the fact that it is not possible to manufacture a single continuous fiber loop, which must be used as the recirculating delay line. The only way the invention of the Suzaki patent could be implemented is to splice a length of glass fiber to produce the loop. Whenever a splice is necessary, there are considerable losses in the light being transmitted through the fiber due to the splice. Therefore, any device embodying a spliced fiber is, and must be, an inefficient device.
A second problem with the Suzaki apparatus is that it does not utilize evanescent field coupling in the coupling device utilized. The coupling device utilized requires that the glass fibers be cut and polished until the cores of the fibers are exposed to create an optical couple between the fibers. This presents the possibility that the core of the fiber may be damaged in the process, further lowering the efficiency of the Suzaki device. In addition, since the evanescent fields are not coupled, the coupling loss, which is the difference between the amount of light going into the coupler and the amount of light coming out of the coupler, may be significant.
Finally, the Suzaki device utilizes multi-mode fibers rather than single mode fibers. Multi-mode fibers have a much larger core diameter than do single mode fibers. Since multi-mode fibers have a larger core, the angle of refraction is greater, and thus multi-mode fibers are quite susceptible to modal dispersion, which will limit the bandwidth to 500 MHZ to 1 GHZ. Even when using a graded index multi-mode fiber, where dispersion is minimized by grading of the index of refraction, maximum band width of signals to be transmitted is only slightly above 1 GHZ. The exact bandwidth, of course, will depend on the frequency of the light, since light of a given wavelength may have minimal dispersion due to properties of the optical fiber.
Single mode fiber does not have modal dispersion problems, since the diameter of the single mode fiber core is relatively small. While any optical fiber will have some material dispersion, the effects of material dispersion are several orders of magnitude less than those of modal dispersion.
Therefore, the Suzaki device possesses the diadvantages of having a limited bandwidth, and of having relatively high losses, which impede the transmission of a pulse train of any length. For these reasons, the Suzaki device is not useful as a recirculating memory device with a high frequency data input.